Compressed air supply systems are used in vehicles, in particular utility vehicles, in order to supply compressed air devices and/or pneumatic systems of the vehicle with compressed air. Such compressed air devices include, for example, a pneumatic suspension unit and a braking unit.
Known compressed air supply systems typically comprise a compressor that provides the compressed air, and an air drying device in which the compressed air provided by the compressor is freed of particles and moisture and/or condensate by air drying material, such as, for example, a filter and a separator, which, if not removed, would otherwise damage the compressed air devices connected thereto and impair the function thereof (for example, the braking action of a braking system).
In such a know pressed air control system, a ventilation control valve to be actuated pneumatically and/or a compressed air control device comprising the ventilation control valve is provided, in order to control operating states of the compressor and the air drying device. Such a ventilation control valve and/or such a compressed air control device is also denoted as a governor. The system pressure provided via the air drying device is guided in a system pressure line to a control inlet of the ventilation control valve. From a predetermined system pressure, the ventilation control valve switches the system pressure via a compressor control line at a control inlet of the compressor and via a ventilation control line at a control inlet of a pneumatic ventilation valve of the air drying device. As a result, firstly, the compressor is switched from a “delivering” operating state into a “holding” operating state, so that the system pressure is not able to be increased further. Secondly, the air drying device is switched from a “delivery” operating state into a “regeneration” operating state, wherein air flows through the air drying material counter to a delivery direction and compressed air is ventilated by the ventilation valve to a vent. In this case, the air drying material are cleaned in a so-called regeneration process and/or flushing process and freed of condensate.
If the system pressure has fallen below a specific limit value, the ventilation control valve switches again so that the ventilation control line and the compressor control line are ventilated by the ventilation control valve to a vent. As a result, ambient air pressure is applied to the control inlets of the compressor and the ventilation valve, wherein the compressor again switches into its “delivering” operating state and the air drying device again switches into its “delivery” operating state.
A drawback with this known compressed air supply system and/or compressed air supply method is that the operating states of the compressor and of the air drying device, depending on a current compressed air requirement of the connected compressed air devices of the vehicle, are adopted at different frequencies and for different lengths of time and thus rather randomly. So-called coasting phases in which the engine driving the compressor is operated without the supply of fuel as a result of the driving mode, for example, when the vehicle is traveling downhill, may not be efficiently utilized in terms of energy. Additionally, after each regeneration of the air drying device, the compressor has to first raise the pressure to the current system pressure in compressed air lines between the compressor and the air drying device and/or the system pressure line and optionally in a compressed air storage tank connected pneumatically to the air drying device, before the system pressure may be increased.
A further compressed air supply system is disclosed in U.S. Pat. No. 6,036,449 A, which differs from the compressed air supply system described above in that an electrically actuatable ventilation control valve is provided instead of the pneumatically actuatable ventilation control valve. As a result, the fixed limits at which the ventilation control valve is switched, are lilted. In particular, an electronic control device may subject the ventilation control line and the compressor control line to the system pressure, or ventilate the two control lines, irrespective of the respective system pressure. The compressed air supply system of U.S. Pat. No. 6,036,449 A thus permits a more flexible control of the regeneration of the air drying device.
The compressed air supply system of U.S. Pat. No. 6,036,449 A, however, has numerous drawbacks. In particular, the compressed air supply system and the above-mentioned compressed air supply system with the ventilation control valves, which may be actuated pneumatically, have a high energy consumption since the compressor has to be operated frequently and for lengthy periods by the engine of the vehicle in order to build an air pressure again (e.g., in the compressed air line that leads from the compressor to the air drying device) to the system pressure after the regeneration of the air control device, before the system pressure may be further increased.